True Atomic-Scale Imaging in Three Dimensions 215
Figure 5. a: Field-ion microscopy (FIM) image displaying a boron layer (bright spots along the dark dashed line). b: Cross-section of the stacking of the FIM micrographs recorded during the field-evaporation sequence. c: Same FIM micrograph stack as in (b) after the curvature correction.
GP zones. More details can be found in the study by Danoix et al. (2012).
Applications to Semiconductors
The main driving force for advanced materials research in the microelectronics industry is the continuous need for scaling down the dimensions of devices. To achieve smaller and better performing devices, their electrical properties have to be accurately engineered. This is achieved by controlling the concentrations and locations of the electro- nically active impurities (dopants) very precisely; ultimately linking device variability to statistical fluctuations in the locations of dopants. Due to the reduced device dimensions, the dopant concentration profiles become confined to features with smaller and smaller dimensions, which can be a few nanometers with very abrupt changes in the local doping concentration (ideally approaching concentration gradients of <1 nm/decade). The fabrication of dopant concentration profiles is hampered first by the physics of the doping processes (such as, statistical fluctuations in the energy loss of ion-implanted dopants, the effects of ion range, and range straggling).The effects of dopant diffusion, clustering, and segregation must be considered during ther- mal treatments, as they lead to deviations from the targeted concentration profiles. The generation of induced clusters, when the local dopant concentration exceeds the solubility limit, can degrade the required electrical properties. Clusters of a few dopant atoms act as charge trapping centers and decrease the electrical performance of the final devices. Progress in semiconductor devices is strongly dependent on the ability to monitor the dopant concentration distributions on the nanometer scale. With its ultimate spatial resolution, FIM is a potentially
suitable technique for studying the incorporation and diffusion of dopants in semiconductors. Boron is a com-
monly used dopant in silicon; as it has a higher evaporation field than silicon, boron atoms appear as bright spots in FIM micrographs of B-doped silicon. Based on this contrast effect, boron atoms can be imaged in silicon by FIM without any ambiguity. As discussed, FIM both provides an unrivaled and near perfect detection efficiency together with a higher
spatial resolution than APT. Hence, applying 3D FIM to different features in silicon, e.g., B-δ-doped layers or B segregation at dislocation loops in a silicon matrix, makes it possible to gain new insights into the interactions of dopant atoms with their host silicon matrix.
Monoatomic B δ-Doped Layers
The depth resolution of FIM can be assessed by analyzing a sample consisting of successive δ boron layers separated by 3 nm of silicon. This structure was covered with a 100-nm thick capping layer of silicon to protect the structure during specimen preparation with a focused ion-beam microscope. The sample was imaged with an FIM using hydrogen as the imaging gas and the specimen was cooled to 80 K. Figure 5a displays an image obtained in the early phase of the measurement. The direction perpendicular to the δ boron layers is the <001> direction of the silicon structure. The spatial resolution along this direction is generally lower than the directions <111> or <011>, where atomic resolu- tion can be achieved as is demonstrated in the next section. In addition, the presence of a high concentration of boron atoms between layers (~1020 boron at/cm3) with a high evaporation field compared with the silicon matrix prevents easy observation of crystallographic features in the FIM images (only the boron atoms are visible in the images). A clear contrast effect corresponding to the highly con- centrated boron δ layer is, however, observed. In Figure 5a the dashed circle connecting the positions of the very bright spots indicates the location of a B δ layer. The planar enriched regions intercept the spherical apex and produce, Figure 5a, a circular contrast effect. The consecutive FIM images taken during the evaporation of the sample were stacked and a cross-sectional view along the depth axis is displayed, Figure 5b. The white curved contours show clearly the location of five δ layers and the curvature induced by the projection of the rounded specimen’s apex onto the planar detector. A corrected reconstruction is shown in Figure 5c, using the previous method, assuming a sample radius of curvature and a constant depth increment between images to flatten the expected layers, which are known to be planar.
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